On my laptop, when I type @us, there is a little box with "username" and when I click on that, after I submit my comment it pings the person. However on my phone this doesn't happen. If I type out @username, does it still ping them? It seems like it doesn't.
Yes, a typed-in at-notification will ping. You could also type it out on your laptop. I frequently do this. (Why do you think it does not?)
The feature you describe is only there to ease the input of a notifying string. For the notification it is ultimately irrelevant how the string came into the input box (auto-complete, typed-in, copy-paste, it does not change a thing); the submitted comment is parsed and processed.
Note that there is no need to type out the full string. The first three letters or any longer starting sub-string (with spaces removed) will do. Note the 'starting' though and note that the string must be an actual substring and not just match the start, so: @Zach will do, yet @Selk won't and @Zack won't either.
[To be complete, let me add that slightly more could be said about the string matching, but for the most part what is between @ and the first subsequent space (or character that cannot be contained in a user name) must be an exact starting substring, only adding a single punctuation is still fine and some special characters are identified.]